The Upper Colorado River Planning Area is characterized by semi-arid to arid alluvial basins with few perennial streams. Anderson, Freethey and Tucci (1992) divided the alluvial basins in south-central Arizona into categories based on similar hydrologic and geologic characteristics. These categories are useful in describing general hydrologic characteristics. The Big Sandy Basin is within two of these four categories, Highland and Southeast basins.

Highland Basins

The aquifers of the Highland Basins, which generally encompass the northeastern portions of the Big Sandy and Bill Williams, consist of hydraulically connected basin fill and younger stream alluvium. These aquifers tend to be discontinuous and limited in extent. Groundwater inflow is from stream channels, mountain front recharge and adjacent consolidated rock aquifers. Groundwater outflow is due to evapotranspiration and baseflow to streams (Anderson, Freethey and Tucci, 1992).

Big Sandy Basin (northeastern portion)

In this portion of the Big Sandy Basin, generally the Fort Rock Sub-basin, (see Figure 4.1-6), the primary hydrologic unit consists of sedimentary rocks composed of Redwall Limestone (a coarse-grained, massive limestone) and the Martin Formation (a fine- to coarse-grained dolomitic limestone). The limestones form a regional aquifer that extends north and east. There is little water development in this portion of the Big Sandy Basin and groundwater flow direction has not been reported. Well yields in three wells varied from 100 to over 1,000 gpm. In this area, water levels were stable in most wells measured between 1990-91 and 2003-04, with water levels ranging from about 130 to 860 feet bls (Figure 4.1-6). Water quality measurements from three wells in the southern portion of the Fort Rock sub-basin showed drinking water exceedences of arsenic and cadmium.

Southeast Basins

Big Sandy Basin (western portion)

With the exception of its northeastern portion, most of the Big Sandy Basin was categorized as a “Southeast Basin” by Anderson, Freethey and Tucci (1992). This area generally corresponds to the Wikieup Sub-basin south of Interstate 10 (see Figure 4.1-6). Southeast Basins are characterized by moderately thick pre-Basin and Range sediments and an overlying layer of lower basin fill to depths of over 1,000 feet. Aquifers generally consist of two or more water-bearing units separated by a fine-grained unit that forms a leaky confining layer over the lower basin fill. Primary water development in the Big Sandy Basin is along the central valley, primarily in upper basin fill that varies from loosely consolidated silty gravel to sandy silt. The floodplain alluvium in the central valley is 30-40 feet thick and is an unconsolidated deposit of gravel and sand. In the Wikieup area, wells greater than 40 feet in depth tap the upper basin fill, which is estimated to be 300 feet deep. North of Wikieup, the upper basin fill is estimated to be 150 to 200 feet deep. Groundwater flow is generally from north to south down the central valley.

Groundwater recharge is estimated at 22,000 AFA and the volume of groundwater in storage is estimated at 9.5 to 21 maf for the entire basin (Table 4.1-6). Median well yield for the entire basin is 300 gpm reported for large (>10-inch) diameter wells and as high as 2,000 gpm at Cane Springs (Figure 4.1-8). Water levels are relatively stable with some declines measured near Wikieup and south of Valentine. Depth to water ranges from 15 feet bls along the Big Sandy River south of Wikieup to over 370 feet along Hackberry Road in the northern part of the Wikieup Sub-basin (Figure 4.1-6). Arsenic, fluoride, lead and radionuclide concentrations that exceed drinking water standards have been measured in wells and springs throughout the western portion of the basin (Figure 4.1-9). Elevated radionuclide and fluoride concentrations are found primarily along the mountain drainages (Cady, 1981).

The U.S. Geological Survey (USGS) divides and subdivides the United States into successively smaller hydrologic units based on hydrologic features. These units are classified into four levels. From largest to smallest these are: regions, subregions, accounting units and cataloging units. A hydrologic unit code (HUC) consisting of two digits for each level in the system is used to identify any hydrologic area (Seaber et al., 1987). A 6-digit unit code corresponds to accounting units, which are used by the USGS for designing and managing the National Water Data Network. (see Figure 4.0-5)

Bill Williams Watershed

The Bill Williams watershed has a drainage area of about 5,393 sq. miles (NEMO, 2005). The watershed drains into Lake Havasu just upstream of Parker Dam near the southern boundary of the planning area. The greatest elevational range in the planning area, from 8,417 feet at Hualapai Peak to 450 feet north of Parker Dam, is found in the watershed. The watershed includes the Bill Williams Basin, most of the Big Sandy Basin and the southern portion of the Sacramento Valley Basin. The watershed is drained by the Bill Williams River and its major tributaries, the Big Sandy and the Santa Maria Rivers and by Burro Creek. A number of perennial streams exist in the watershed including segments of the Big Sandy River, the Bill Williams River, Burro Creek, Kirkland Creek, Date Creek, the Santa Maria River, and Trout Creek. Numerous intermittent streams are also present (Figures 4.1-5 and 4.2-5)

Construction of Alamo Dam on the Bill Williams River in 1968 significantly impacted streamflow below the dam. Built as a flood-control structure, the dam is now operated in a manner to benefit downriver wildlife refuges and vegetation along the river. According to NEMO (2005), 185 miles of perennial streamflow exist in the watershed, mostly restricted to the main stem of the Bill Williams River. Water levels in the Bill Williams River below Alamo Dam are affected by the water levels in Lake Havasu. Alamo Lake is the largest lake in the watershed with about 13,400 acres of open water surface. Prior to dam construction the Bill Williams River produced some of the largest floods in Arizona history, with a peak discharge (200,000 ft3/s) comparable to the largest known Colorado River floods (Webb and others, 2007).

Bill Williams River near the confluence with the Colorado River, Bill Williams Basin. Construction of Alamo Dam in 1968 significantly impacted streamflow below the dam. The dam is now operated in a manner to benefit downriver wildlife refuges and vegetation along the river.

Median annual streamflow in the Bill Williams River below Alamo Dam is about 34,000 acre-feet, but a maximum annual flow of almost 702,000 acre-feet was recorded in 1993. By comparison, the median annual flow at a gage on the Santa Maria River upstream of the dam is about 10,000 af. The median annual flow recorded at a gage south of Wikieup on the other major tributary to the Bill Williams River, the Big Sandy River, is about 27,000 af.

Within the watershed, perennial streams originate from spring discharges from crystalline rocks. Most of the public water supply for the town of Bagdad comes from spring flow that discharges to Francis Creek, a tributary to Burro Creek. Twelve large springs have been identified in the watershed; the largest is located in the Big Sandy Basin where discharge from an unnamed spring south of Cane Springs measured 1,600 gpm. The largest spring in the Bill Williams Basin was measured at 228 gpm. There are no large springs reported in the Sacramento Valley Basin portion of the watershed. Most springs are located in the vicinity of Valentine, along the Big Sandy River, and near the eastern boundary of the Bill Williams Basin. All measurements were taken prior to 1980 and some measurements are as old as 1943; therefore, the reported discharges may no longer be representative of current conditions.